Ans. c (Activated protein 'C') (Ref. Harper's biochemistry 27th ed., Chapter 50)Patients with factor V Leiden (which has a glutamine residue in place of an arginine at position 506) have a T risk of venous thrombotic disease because factor V Leiden is resistant to inactivation by activated protein C (APC).This condition is APC resistance.# Thrombin is involved in an regulatory mechanism that operates in coagulation. It combines with thrombomodulin, a glycoprotein present on the surfaces of endothelial cells.# The complex activates protein C.# In combination with protein S, APC degrades factors Va and Villa, limiting their actions in coagulation.The Functions of the proteins involved in blood coagulationZymogens of serine proteasesFactor XIIBinds to negatively charged surface, eg, kaolin, glass; activated by high-MW kininogen and kallikreinFactor XIActivated by factor XIIaFactor IXActivated by factor XIa in presence of Ca2+Factor VHActivated thrombin in presence of Ca2+Factor XActivated on surface of activated platelets by tenase complex (Ca2+, factors Villa and IXa) and by factor VIIa in presence of tissue factor and Ca2+Factor IIActivated on surface of activated platelets by prothrombinase complex (Ca2+, factors Va and Xa) (Gla = g -carboxyglutamate)Factor VIIIActivated by thrombin; factor Villa is a cofactor in the activation of factor X by factor IXaFactor VActivated by thrombin; factor Va is a cofactor in the activation of prothrombin by factor XaTissue factor (factor III)A glycoprotein expressed on the surface of stimulated endothelial cells and monocytes to act as a cofactor for factor VIIaFibrinogenFactor ICleaved by thrombin to form fibrin clotThiol-dependent transglutaminaseFactor XIIIActivated by thrombin in presence of Ca2+; stabilizes fibrin clot by covalent cross-linkingRegulatory and other proteinsProtein CActivated to protein Ca by thrombin bound to thrombomodulin; then degrades factors Villa and VaProtein SActs as a cofactor of protein C; both proteins contain Gla (y -carboxyglutamate) residuesThrombomodulinProtein on the surface of endothelial cells; binds thrombin, which then activates protein C